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| United States Patent |
5,098,326
|
|
Gorczyca
, et al.
|
March 24, 1992
|
Method for applying a protective coating to a high-intensity metal
halide discharge lamp
Abstract
A method for applying a protective coating to the inner surface of the arc
tube of a high-intensity metal halide discharge lamp involves dosing the
arc tube with an inert gas that is doped with a metal hydride gas.
Preferably, the metal hydride gas comprises silane. The arc tube is heated
to a sufficiently high temperature to decompose the silane gas. As a
result, silicon is deposited as a protective coating on the inner surface
of the arc tube wall. The hydrogen gas that is generated by the silane
decomposition is removed from the system either by pumping it out before
dosing the arc tube with the final arc tube fill, or by diffusion through
the arc tube wall during operation of the lamp.
| Inventors:
|
Gorczyca; Thomas (Schenectady, NY);
Witting; Harald L. (Burnt Hills, NY)
|
| Assignee:
|
General Electric Company (Schenectady, NY)
|
| Appl. No.:
|
627110 |
| Filed:
|
December 13, 1990 |
| Current U.S. Class: |
445/26; 427/107; 445/58 |
| Intern'l Class: |
H01J 009/20 |
| Field of Search: |
445/10,17,26,18,58
427/107,124,252
|
References Cited
U.S. Patent Documents
| 2183302 | Dec., 1939 | Brauer | 445/10.
|
| 3729335 | Apr., 1973 | Domrachev et al. | 427/252.
|
| 4374157 | Feb., 1983 | Barbier et al. | 427/107.
|
| 4810938 | Mar., 1989 | Johnson et al. | 315/248.
|
| 4812702 | Mar., 1989 | Anderson | 313/153.
|
| 4857097 | Aug., 1989 | Berry | 427/252.
|
| 4972120 | Nov., 1990 | Witting | 313/638.
|
| Foreign Patent Documents |
| 269558 | Nov., 1927 | GB | 427/124.
|
| 1248747 | Oct., 1971 | GB | 427/252.
|
Other References
Waymouth, John F., Electric Discharge Lamps, M.I.T. Press 1971, pp.
266-277.
|
Primary Examiner: Rowan; Kurt
Assistant Examiner: Knapp; Jeffrey T.
Attorney, Agent or Firm: Breedlove; Jill M., Davis, Jr.; James C., Snyder; Marvin
Claims
What is claimed is:
1. A method for applying a protective metal coating to the inner surface of
the arc tube of a high-intensity discharge lamp, comprising the steps of:
filling said arc with an inert gas that is doped with a metal hydride of a
predetermined quantity;
heating said arc tube at a sufficiently high temperature for a sufficiently
long period of time to decompose said metal hydride in order to form a
metal coating on the inner surface of said arc tube;
evacuating said arc tube; and
filling said arc tube with a solid dose including at least one metal halide
and with a gaseous dose including at least one inert gas; and
sealing said arc tube.
2. The method of claim 1 wherein the step of heating said arc tube
comprises heating in an oven.
3. The method of claim 1 wherein the step of heating said arc tube
comprises operating the lamp.
4. The method of claim 1 wherein said metal hydride comprises silane and
said coating comprises silicon.
5. The method of claim 4 wherein said temperature is in the range from
approximately 500.degree. C. to 900.degree. C.
6. The method of claim 5 wherein said temperature is approximately
550.degree. C.
7. The method of claim 1 wherein said protective coating has a thickness in
the range from approximately 3 to 40 nanometers.
8. The method of claim 7 wherein said protective coating has a thickness in
the range from approximately 10 to 20 nanometers.
9. A method for applying a protective metal coating to the inner surface of
the arc tube of a high-intensity discharge lamp, comprising the steps of:
filling said arc tube with a gaseous dose, including at least one inert gas
and a metal hydride, and with a solid dose, including at least one metal
halide, to a predetermined pressure;
sealing said arc tube; and
heating said arc tube at a sufficiently high temperature for a sufficiently
long period of time to decompose said metal hydride in order to form a
metal coating on the inner surface of said arc tube and to allow hydrogen
generated from the decomposition of said metal hydride to diffuse from
said arc tube.
10. The method of claim 9 wherein the step of heating said arc tube
comprises heating in an oven.
11. The method of claim 9 wherein the step of heating said arc tube
comprises operating the lamp.
12. The method of claim 9 wherein said metal hydride comprises silane and
said coating comprises silicon.
13. The method of claim 12 wherein said temperature is in the range from
approximately 500.degree. C. to 900.degree. C.
14. The method of claim 13 wherein said temperature is approximately
550.degree. C.
15. The method of claim 9 wherein said protective coating has a thickness
in the range from approximately 3 to 40 nanometers.
16. The method of claim 15 wherein said protective coating has a thickness
in the range from approximately 10 to 20 nanometers.
Description
FIELD OF THE INVENTION
The present invention relates generally to high-intensity metal halide
discharge lamps. More particularly, the present invention relates to a
method for applying a protective coating to the inner surface of the arc
tube of such a lamp.
BACKGROUND OF THE INVENTION
In operation of a high-intensity metal halide discharge lamp, visible
radiation is emitted by the metallic portion of the metal halide fill at
relatively high pressure upon excitation typically caused by passage of
current therethrough. One class of high-intensity metal halide lamps
comprises electrodeless lamps which generate an arc discharge by
establishing a solenoidal electric field in the high-pressure gaseous lamp
fill comprising the combination of one or more metal halides and an inert
buffer gas. In particular, the lamp fill, or discharge plasma, is excited
by radio frequency (RF) current in an excitation coil surrounding an arc
tube which contains the fill. The arc tube and excitation coil assembly
acts essentially is a transformer which couples RF energy to the plasma.
That is, the excitation coil acts as a primary coil, and the plasma
functions as a single-turn secondary. RF current is the excitation coil
produces a time-varying magnetic field, in turn creating an electric field
in the plasma which closes completely upon itself, i.e., a solenoidal
electric field. Current flows as a result of this electric field, thus
producing a toroidal arc discharge in the arc tube.
High-intensity, metal halide discharge lamps, such as the aforementioned
electrodeless lamps, generally provide good color rendition and high
efficacy in accordance with the principles of general purpose
illumination. However, the lifetime of such lamps can be limited by the
loss of the metallic portion of the metal halide fill during lamp
operation and the corresponding buildup of free halogen. In particular,
the loss of the metal atoms shortens the useful life of the lamp by
reducing the visible light output. Moreover, the loss of the metal atoms
leads to the release of free halogen into the arc tube, which may cause
arc instability and eventual arc extinction, especially in electrodeless
high-intensity metal halide discharge lamps.
The loss of the metallic portion of the metal halide fill may be
attributable to the electric field of the arc discharge which moves metal
ions to the arc tube wall. For example, as explained in Electric Discharge
Lamps by John F. Waymouth, M.I.T. Press, 1971, pp. 266-277, in a
high-intensity discharge lamp containing a sodium iodide fill, sodium
iodide is dissociated by the arc discharge into positive sodium ions and
negative iodine ions. The positive sodium ions are driven towards the arc
tube wall by the electric field of the arc discharge. Sodium ions which do
not recombine with iodine ions before reaching the wall may react
chemically at the wall, or they may pass through the wall and then react
outside the arc tube. (Normally, there is an outer light-transmissive
envelope disposed about the arc tube.) These sodium ions may react to form
sodium silicate or sodium oxide by reacting with a silica arc tube or with
oxygen impurities. As more and more sodium atoms are lost, light output
decreases, and there is also a buildup of free iodine within the arc tube
that may lead to arc instability and eventual arc extinction. Furthermore,
the arc tube surface may degrade as a result of the ion bombardment.
As described in commonly assigned, copending U.S. patent application of
Witting et al., entitled "Protective Coating for High-Intensity Metal
Halide Discharge Lamps", Ser. No. 553,304, filed July 16, 1990, now
allowed a suitable protective coating comprises, for example, a silicon
layer which is sufficiently thick to prevent a substantial loss of the
metallic component of the metal halide fill, but which is also
sufficiently thin so as to allow only minimal blockage of visible light
output from the arc tube. According to the cited patent application, which
is incorporated by reference herein, one method of applying the protective
coating involves a chemical vapor deposition process wherein the coating
is initially applied to both the inner and outer surfaces of the arc tube,
the outer coating being subsequently removed by immersing the arc tube in
a suitable etchant.
Although the method of the hereinabove cited Witting et al. patent
application, Ser. No. 553,304, is effective in applying a protective
coating to arc tubes of high-intensity metal halide discharge lamps, it
may be desirable to provide a simpler method of applying such a coating,
thereby simplifying the lamp manufacturing process.
Accordingly, an object of the present invention is to provide a new and
improved method for applying a protective coating to the inner surface of
an arc tube of a high-intensity metal halide discharge lamp.
SUMMARY OF THE INVENTION
The foregoing and other objects of the present invention are achieved in a
method for applying a protective coating to the inner surface of the arc
tube of a high-intensity metal halide discharge lamp which involves dosing
the arc tube with an inert gas that is doped with a metal hydride gas.
Preferably, the metal hydride gas comprises silicon hydride, or silane.
The silane gas is decomposed into silicon and hydrogen by exposing the arc
tube to a temperature of approximately 550.degree. C., either by heating
in an oven or by driving a discharge in the arc tube. As a result, silicon
is deposited as a protective coating on the inner surface of the arc tube
wall. The hydrogen gas generated by the silane decomposition is removed
from the arc tube either by pumping it out before dosing the arc tube with
its final fill, or by gradual diffusion through the arc tube wall during
lamp operation.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become apparent
from the following detailed description of the invention when read with
the sole accompanying drawing FIGURE which illustrates a high-intensity
metal halide discharge lamp having a protective coating of a type
described herein.
DETAILED DESCRIPTION OF THE INVENTION
The sole drawing FIGURE illustrates a high-intensity, metal halide
discharge lamp 10 employing a protective coating 12 in accordance with the
present invention. For purposes of illustration, lamp 10 is shown as an
electrodeless high-intensity metal halide discharge lamp. However, it is
to be understood that the principles of the present invention apply
equally well to high-intensity metal halide discharge lamps having
electrodes. As shown, electrodeless metal halide discharge lamp 10
includes an arc tube 14 formed of a high temperature glass, such as fused
silica, or an optically transparent ceramic, such as polycrystalline
alumina. By way of example, arc tube 14 is shown as having a substantially
ellipsoid shape. However, arc tubes of other shapes may be desirable,
depending upon the application. For example, arc tube 14 may be spherical
or may have the shape of a short cylinder, or "pillbox", having rounded
edges, if desired.
Arc tube 14 contains a metal halide fill in which a solenoidal arc
discharge is excited during lamp operation. A suitable fill, described in
commonly assigned U.S. Pat. No. 4,810,938 of P. D. Johnson, J. T. Dakin
and J. M. Anderson, issued on Mar. 7, 1989, comprises a sodium halide, a
cerium halide and xenon combined in weight proportions to generate visible
radiation exhibiting high efficacy and good color rendering capability at
white color temperatures. For example, such a fill according to the
Johnson et al. patent may comprise sodium iodide and cerium chloride, in
equal weight proportions, in combination with xenon at a partial pressure
of about 500 torr. The Johnson et al. patent is incorporated by reference
herein. Another suitable fill is described in commonly assigned U.S. Pat.
No. 4,972,120, issued Nov. 20, 1990 to H. L. Witting, which patent is
incorporated by reference herein. The fill of Witting U.S. Pat No.
4,972,120 comprises a combination of a lanthanum halide, a sodium halide,
a cerium halide and xenon or krypton as a buffer gas. For example, a fill
according to the Witting patent may comprise a combination of lanthanum
iodide, sodium iodide, cerium iodide, and 250 torr partial pressure of
xenon.
Electrical power is applied to the HID lamp by an excitation coil 16
disposed about arc tube 14 which is driven by an RF signal via a ballast
18. A suitable excitation coil 16 may comprise, for example, a two-turn
coil having a configuration such as that described in commonly assigned,
copending U.S patent application of G. A. Farrall, Ser. No. 493,266, filed
Mar. 14, 1990, now allowed which patent application is incorporated by
reference herein. Such a coil configuration results in very high
efficiency and causes only minimal blockage of light from the lamp. The
overall shape of the excitation coil of the Farrall application is
generally that of a surface formed by rotating a bilaterally symmetrical
trapezoid about a coil center line situated in the same plane as the
trapezoid, but which line does not intersect the trapezoid. However, other
suitable coil configurations may be used, such as that described in
commonly assigned U.S. Pat. No. 4,812,702 of J. M. Anderson, issued Mar.
14, 1989, which patent is incorporated by reference herein. In particular,
the Anderson patent describes a coil having six turns which are arranged
to have a substantially V-shaped cross section on each side of a coil
center line. Still another suitable excitation coil may be of solenoidal
shape, for example.
In operation, RF current in coil 16 results in a time-varying magnetic
field which produces within arc tube 14 an electric field that completely
closes upon itself. Current flows through the fill within arc tube 14 as a
result of this solenoidal electric field, producing a toroidal arc
discharge 20 in arc tube 14. The operation of an exemplary electrodeless
HID lamp is described in Johnson et al. U.S. Pat. No. 4,810,938, cited
hereinabove.
The protective coating 12 on the inner surface of arc tube 14 is of
sufficient thickness to prevent a substantial loss of the metallic portion
of the metal halide fill and hence a corresponding substantial buildup of
free halogen. In addition, the protective coating is sufficiently thin to
allow only minimal blockage of visible light output from the arc tube.
Advantageously, since the metallic portion of the fill generates the
visible radiation during lamp operation, the useful life of the lamp is
extended by preventing a substantial loss thereof. Furthermore, since a
buildup of free halogen typically causes arc instability and eventual arc
extinction, preventing such a buildup likewise extends the useful life of
the lamp.
In a preferred embodiment of lamp 10, as described in Witting et al. U.S.
patent application, Ser. No. 553,304, cited hereinabove, arc tube 14 is
comprised of fused silica, and protective coating 12 comprises a layer of
silicon. A preferred thickness of silicon coating 12 is between 3 and 40
nanometers, with a more preferred range being from 10 to 20 nanometers.
Silicon is a preferred protective coating because it has a relatively low
thermal expansion coefficient and a high melting point. In addition,
silicon may be advantageously employed as a coating on fused silica arc
tubes because it is chemically compatible with silica and because it
reacts with oxygen impurities to form silica. Moreover, for metal halide
lamps having sodium as one of the fill ingredients, silicon is a preferred
coating because it is a poor solvent for sodium and does not form
compounds therewith.
In accordance with a preferred embodiment of the present invention, silicon
coating 12 is applied to the inner surface of arc tube 14 by filling the
arc tube with an inert gas that is doped with silicon hydride, or silane,
and then heating the arc tube for a suitable time period, e.g. 1 to 90
minutes, in the range from approximately 500.degree. C. to 900.degree. C.
Heating of the arc tube can be accomplished either by heating in an oven
or by driving a discharge in the arc tube, or a combination thereof. In
particular, heating of the arc tube in an oven causes the silane gas to
decompose thermally. On the other hand, driving a discharge in the arc
tube results in both thermal and plasma decomposition of the silane gas.
In either case, however, silane decomposition causes nucleation and
deposition of silicon coating 12 on the inner surface of arc tube 14. The
total silicon content inside the arc tube and the resulting average
silicon coating thickness are determined by the partial pressure of the
silane gas. Those skilled in the art of chemical vapor deposition will
recognize that the coating time can be reduced by increasing the coating
temperature and that the coating temperature can be reduced if the coating
time is increased. Moreover, if heating is accomplished by driving a
discharge, the plasma decomposition further reduces the coating time.
After coating 12 has been applied to the inner surface of arc tube 14, the
arc tube is evacuated in order to remove the hydrogen that was generated
by the dissociation of the silane. The arc tube is then filled with a
typical dose of at least one metal halide and at least one inert gas, and
finally sealed.
EXAMPLE
A fused silica arc tube of spherical shape having an inside volume of 3
cubic centimeters is filled with a 5% silane-doped inert gas to a total
pressure of 250 torr and is heated for 5 minutes in an oven at
approximately 550.degree. C. As a result, approximately 0.05 milligram of
silicon is deposited as a coating having a thickness of approximately 20
nanometers on the inner surface of the arc tube. The arc tube is then
evacuated in order to remove the hydrogen that was generated by the
dissociation of the silane. The arc tube is then filled with a solid dose
of 4.75 milligrams of sodium iodide and 2.25 milligrams of cerium iodide,
and also with a gaseous dose of krypton, and finally sealed.
An alternative method of the present invention involves adding the silane
gas directly to the arc tube fill which typically includes at least one
metal halide and at least one inert gas. The arc tube is sealed and then
heated either in an oven or by driving a discharge in the arc tube, or by
a combination thereof. As a result, silicon coating 12 is deposited on the
inner surface of the arc tube wall. The hydrogen gas that is generated by
the decomposition of silane is removed by diffusion through the hot arc
tube wall. Hydrogen diffuses through hot silica at a relatively fast rate
due to its small atomic diameter.
EXAMPLE
A fused silica arc tube of spherical shape having an inside volume of 3
cubic centimeters is filled with a solid dose of 4.75 milligrams of sodium
iodide and 2.25 milligrams of cerium iodide, and with a gas dose of 95%
krypton and 5% silane at a total pressure of 250 torr. The arc tube is
sealed and then heated for 30 minutes at approximately 550.degree. C. As a
result, silicon in the quantity of approximately 0.05 milligram is
deposited on the inner surface of the arc tube as a coating having a
thickness of approximately 20 nanometers. A discharge is then driven in
the arc tube for an additional 60 minutes. The discharge heats the arc
tube to a temperature of approximately 800.degree. C. and allows free
hydrogen to diffuse through the arc tube wall.
Although the method of the present invention has been described in detail
with reference to a silicon coating, it is to be understood that the
method of the present invention may be used to apply to high-intensity
metal halide discharge lamps other suitable protective coatings
comprising, for example, other metals or metal silicates.
While the preferred embodiments of the present invention have been shown
and described herein, it will be obvious that such embodiments are
provided by way of example only. Numerous variations, changes and
substitutions will occur to those of skill in the art without departing
from the invention herein. Accordingly, it is intended that the invention
be limited only by the spirit and scope of the appended claims.
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